Search for displaced dimuons in proton-proton collisions at $\sqrt{s}$ = 13 TeV with the CMS muon system

Long-lived particles (LLPs) naturally appear in many well-motivated theories beyond the Standard Model of Particle Physics (SM). These LLPs could be produced in high-energy particle collisions and subsequently decay into known particles, which can be detected a measurable distance away from the LLP point of creation. This work presents an inclusive search for neutral, exotic LLPs in events with pairs of oppositely charged muons originating from a common displaced vertex. The underlying dataset was recorded by the CMS detector during the LHC Run 2 and comprised an integrated luminosity of 97.6 fb$^{-1}$ of proton-proton collisions at a center-of-mass energy of $\sqrt{s}$ = 13 TeV. The search is sensitive to LLP decays beyond the CMS tracking system because it uses muons reconstructed by the CMS muon detectors only. To measure the relevant muon trigger and reconstruction efficiencies as a function of displacement and to assess systematic uncertainties and scale factors in the analysis, performance studies were carried out with cosmic-ray muons recorded by CMS in 2016. The results are interpreted in the context of two benchmark models: (1) a scenario in which the final-state muons are the decay products of a generic long-lived, scalar boson from anon-SM Higgs initial state (“BSM heavy scalar“ model), and (2) the “Hidden Abelian Higgs model“ which features long-lived, scalar “dark photons“ as the intermediate LLP states giving rise to displaced dimuons in the final state. Upper limits on the production cross-sections in both interpretations are presented for a wide range of LLP mass-lifetime hypotheses. In all scenarios, there is good agreement between expected and observed upper limits and no statistically significant excess over the predicted SM background is reported.